(a) Field of the Invention
The present invention relates to a zoom lens. More specifically, the present invention relates to a zoom lens for cameras using an image pickup device such as a charge-coupled device (CCD).
(b) Description of the Related Art
Recently, the use of electronic still cameras and video cameras employing a CCD or a solid-state image pickup device has been expanding rapidly, while the needs for miniaturization and weight/cost savings have also been increasing. Miniaturization and weight/cost savings are needed for zoom lenses that are built into cameras.
An optical system using a solid-state image pickup device employs a crystal filter so as to prevent moiré caused by the periodical structure of the image pickup device. Therefore, the thickness and location of the crystal filter must be taken into consideration in designing optical systems to secure a long retrofocal length. In particular, the telecentricity of an incident ray to the image side to face an image-forming plane is an important factor of the design.
Conventional optical systems using a solid-state image pickup device include: Japanese Patent Application Laid-open No. SHO 56-123512, Japanese Patent Application Laid-open No. SHO 63-292106, Japanese Patent Application Laid-open No. PYUNG 6-94996, Japanese Patent Application Laid-open No. PYUNG 5-173071, Japanese Patent Application Laid-open No. PYUNG 11-84243; and Japanese Patent Application Laid-open No. 2002-14284.
The zoom lens disclosed in Japanese Patent Application Laid-open No. SHO 56-123512 comprises, in order from the object side, a first lens group having a negative refractive power, and a second lens group having a positive refractive power. The second lens group is moved to perform zooming. The first lens group is moved to compensate for the movement of an image point caused by the zooming. The first and second lens groups are moved along the optical axis during the zooming, so the overall length of the optical system is greatly changed to increase the structure complexity of the lens barrel. This makes it difficult to achieve a zoom ratio of greater than 2× and particularly to realize miniaturization of the optical system.
The zoom lens disclosed in Japanese Patent Application Laid-open No. SHO 63-292106, is a lens system that does not change its overall length while zooming. This lens system may include a first lens group having a negative refractive power and remaining fixed while zooming, a second lens group having a positive refractive power and moving along the optical axis while zooming, and a third lens group having a positive refractive power. The second and third lens groups are moved towards the object side while zooming from the wide-angle position to the telephoto position, so the overall length of the optical system must be increased to achieve a performance correction at a wide angle. This increases the size of the optical system, and the miniaturization of the system becomes difficult.
The zoom lens disclosed in Japanese Patent Application Laid-open No. PYUNG 6-94996 is a three-group zoom lens that includes a third lens group having a negative or positive refractive power on a second lens group to enhance the zoom ratio and realize a compact optical system in the two-group zooming system. In the zoom lens, the first and second lens groups move along the optical axis to perform a zooming with the third lens group fixed. However, this may change the overall length of the optical system and attain a zoom ratio of only about 2×.
The zoom lens disclosed in Japanese Patent Application Laid-open No. PYUNG 5-173071 comprises, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, a third lens group having a negative refractive power, and a fourth lens group having a positive refractive power. The respective lens groups are moved while zooming, so the configuration of the lens barrel is too complex.
The zoom lens disclosed in Japanese Patent Application Laid-open No. PYUNG 11-84243 comprises, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. The zooming is performed in the direction of decreasing the distance between the first and second lens groups and the distance between the second and third lens groups. In this structure, the distance between the second and third lens group decreases towards the telephoto position, so the focusing according to the distance from the object must be performed with the first or second lens group. But, the number of lenses to be moved during the focusing is increased, because the first and second lens groups have many more lenses than the third lens group. This increases focusing errors, and makes the miniaturization of the optical system more difficult due to the large size of the movable parts.
The zoom lens disclosed in Japanese Patent Application Laid-open No. 2002-14284 comprises a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. In the zoom lens, the first, second and third lens groups are moved to perform a zooming, achieving a zoom ratio of 2× to 3× with 5 to 8 lenses. However, the number of lenses increases with an increase in the zoom ratio, making it difficult to attain a miniature zoom lens with a high zoom ratio.